This article discusses the difference between active and passive biasing circuits used in buyer audio products. The utilization of these circuits in digital potentiometers and volume controls is evaluated in terms of audio parameters like mute and separation. Mathematical equations help designers assess their design needs for a specific situation.

Since designers are pressed to reduce cost for consumer products. While using digital potentiometer and volume-control ICs in its place of mechanical potentiometers allow a wider selection of user interfaces for the customer, these ICs need specific concern to circuit details for achieving best audio performance. The most is trading off performance with cost.

This article will focus on a single-supply purpose, which is the widely used in battery or wall adapter powered products. In a single-supply application all parts of the system are run with VDD and ground, and signals move between VDD and ground. Capacitors are used between stages or eliminated due to cost and performance. The wiper buffer lessens the current passing the array of switches to lessen their distortion. This article looks at how the bias generator influences the circuit's performance.

A digital potentiometer is a group of switches and resistors controlled by logic that imitates a mechanical potentiometer's sliding contact wiper. A volume-control IC is distinguished from the digital potentiometer by the integration of two circuits that are vital for best audio performance. The wiper buffer is an op amp and the bias generator which is the source of VBIAS.

If a digital potentiometer is used and keeping the cost minimal, the bias voltage can be generated with a passive resistive network. The resistor values are typically equal in value to set VBIAS at the midpoint of VDD and ground. To reduce AC impedance and clean up any noise, a bypass capacitor is added.

The passive circuits give poor performance and add both cost and size. An dynamic circuit that makes a buffered version of the resistor-divider works well, but adds cost of an op amp. A volume-control IC like the MAX5486 is an all-in-one solution that includes this function together with a volume control with a VBIAS and wiper buffers needed for audio applications digital potentiometer in a single package. Other Maxim products of volume-control ICs provide ways that can be directly interfaced to a microprocessor, rotary encoders, pushbuttons or infrared remote controls. Additional characteristics like zero crossing synchronized wiper movements makes them highly recommended for audio applications.

http://www.maxim-ic.com/appnotes.cfm/an_pk/4262